One of the major challenges facing the VSTOL aircraft designer has been to design a propulsion system which would yield vertical thrust centered about the aircraft longitudinal center of gravity while still allowing the pilot to be located up front for good visibility, the engine to be aft of the center of gravity for longitudinal weight balance and the engine to exhaust directly aft without impinging on the airframe. Horizontal hover balance cannot be achieved by simple downward deflection of the thrust from an aft located engine. A nose down pitching moment results and must be balanced by a downward thrust forward of the aircraft center of gravity.
Some proposed methods of of supplying this forward balancing thrust are the use of vertically thrusting lift engines or lift fans which are stowed during horizontal flight and thus add nothing to the cruise performance of the aircraft. These lift engines or lift fans must be either redundant or interconnected with the main engines to prevent pitch imbalance in the event of an engine failure. In the case of the lift engines, multiple engines have been utilized to achieve redundancy. With fans and propellers, hot gas ducting or gears and shafts are used to drive them and thus provide the necessary interconnect. Because the lift fans are stowed in flight, these interconnects must be broken for cruise and reengaged for landing. Thus, large horsepower clutches or hot gas diverters are required.
An aircraft currently in production that can be used as an example of the prior art is the AV-8A Harrier VTOL aircraft. The Harrier utilizes a fan jet engine located approximately at the aircraft center of gravity for both hover and cruise propulsion. The fan air is collected and discharged forward of the center of gravity and the jet thrust is discharged aft of the center of gravity. The exhaust gases are discharged through swivel nozzles out of both sides of the engine either down for vertical lift or aft for cruise. The fan air thrust balances the jet thrust about the aircraft center of gravity in hover. Because the engine is located on the center of gravity, aircraft structure and equipment must be located aft of the engine to balance the pilot compartment forward of the engine. Afterburning with this configuration has proven difficult because it must be achieved inside the swivel nozzle. The fan air and the turbine exhaust gases are maintained in separate flow paths through all modes of flight.
U.S. Pat. No. 2,912,188 to D. E. Singelmann, et al., issued Nov. 10, 1959 also discloses a VTOL aircraft using a tiltable combustion chamber which permits thrust to be directed downwardly for vertical take off and loading. However, for cruise flight, these tiltable chambers are rotated aft but the thrust is discharged separately from that produced aft of the center of gravity of the craft through separate turbines. This structure is complex and does not permit the reduction of the frontal area of the craft as is provided in the present invention.